The present invention relates to a method for inhibiting corrosion of aluminum and its alloys when placed in contact with strong sulfuric acid solutions. More particularly, the present invention relates to dissolving corrosion inhibiting amounts of fluoride containing compounds in sulfuric acid solutions containing from about 0-20 wt. percent water, thereby inhibiting corrosion of aluminum metal in contact with such sulfuric acid solutions.
It is well-known that aluminum and aluminum alloys are subject to corrosion when placed in contact with strong sulfuric acid solutions. For this reason, aluminum is not commonly used as a material of construction in situations where prolonged contact of the aluminum with strong sulfuric acid solutions may be expected.
One particular application wherein materials of construction are subjected to prolonged contact with strong sulfuric acid solutions is in a sulfuric acid catalyzed light olefin alkylation process. In such a process, carbon steel is commonly employed as a material of construction and shows good corrosion resistance of sulfuric acid solutions of from about 87 to 98 weight percent H.sub.2 SO.sub.4 at temperatures below about 40.degree.C. The corrosion resistance of carbon steel to acids other than H.sub.2 SO.sub.4 and at temperatures above about 40.degree.C may decrease sharply. For instance, in processes for alkylating light olefins with isoparaffin hydrocarbons, such as disclosed in U.S. Pat. No. 3,231,633, wherein the acid catalyst comprises a mixture of sulfuric acid and fluorosulfuric acid, carbon steel may undergo moderate to severe corrosion in the presence of such mixed acid catalyst. Particularly, in alkylation processes employing acid catalysts comprising mixtures of sulfuric and fluorosulfonic acid, the fluorosulfonic acid must be removed from the spent acid catalyst prior to regeneration of the remaining spent sulfuric acid. One means for separating fluorosulfonic acid is to treat the spent catalyst at an elevated temperature in the neighborhood of 100.degree.C at a reduced pressure. Under these conditions the fluorosulfonic acid breaks down to form hydrogen fluoride which is recovered as a vapor. At elevated temperature, such as 100.degree.C, carbon steel may undergo moderate to severe corrosion in the presence of either strong sulfuric acid solutions or acid mixtures of sulfuric acid and fluorosulfonic acid. Consequently, other materials of construction than carbon steel must be used in processes and systems wherein such material will be in prolonged contact with mixtures of sulfuric acid and fluorosulfonic acid or with strong sulfuric acid solutions at elevated temperatures.
Specially formulated alloys, such as Hastelloy C and Monel 400 are known to have superior corrosion resistance in acid environments; additionally, lead is commonly used in an environment where contact with sulfuric acid may be expected. These metals are, however, expensive and difficult to fabricate.